Two major goals of developmental neurobiology are to identify the germinal cells which form the central nervous system and to characterize the cellular and molecular mechanisms by which these cells generate the proper numbers and types of neurons and glial cells. Recently, retroviral and genetic studies in mice have demonstrated that radial glial cells (RGCs) contribute significantly to neocortical development by generating postmitotic neurons and then serving as the substrate for the migration of those daughter cells into the neocortical wall. Furthermore, RGCs are thought now to be the prenatal stem cell of the neocortex, and several groups have suggested that the neocortical ventricular zone (VZ) is composed solely of RGCs during embryonic neurogenesis. However, it remains unclear how a single VZ precursor cell type can generate the vast diversity of neocortical neurons. In addition, since multiple types of precursors have been found in the human and monkey VZ, whether this heterogeneity in the primate VZ represents evolutionary divergence of rodents and primates also remains a question. We have uncovered significant precursor diversity in the in vivo murine VZ. Through the use of multiple histological, genetic and ultrastructural techniques - many of which we innovated for this project - we now show that RGCs are joined by another type of resident VZ cell which we have named the short neural precursor cell (SNP). We demonstrate that RGCs and SNPs can be distinguished morphologically, molecularly, and with respect to proliferation kinetics and lineage potential. In addition, we have uncovered in vivo differences between classes of RGCs which suggest that not all RGCs are multipotent stem cells. Taken together, our published and Preliminary Data fundamentally alter our understanding of the composition of the mammalian VZ and clearly indicate that diversity in this germinal compartment is required for proper neocortical growth and function. This project will comprehensively test the overall hypothesis that the VZ becomes heterogeneous during embryonic development through diversification of a common VZ ancestor and that this complexity is necessary for proper neocortical growth.